A miniaturized fiber optic hydrophone (FOH) based on a composite metal diaphragm with an air back cavity and a high finesse extrinsic Fabry-Perot interferometric (EFPI) scheme for low-frequency underwater acoustic sensing is proposed and experimentally demonstrated in this paper. A composite metal diaphragm is used to improve the stability of the hydrophone. A balance channel is used to equilibrate the hydrostatic pressure and maintain an air cavity, which improves the mechanical sensitivity. In addition, a white light interferometry (WLI) phase demodulation is used to demodulate the high finesse interferometer consisted of the fiber collimator end face and the diaphragm, which improves the phase sensitivity. Experimental results show that the enhanced phase sensitivity of the hydrophone is about -122.5 dB re 1 rad/µPa @ 200 Hz and the sensitivity fluctuation is below 2.5 dB between 3 Hz and 400 Hz, while the minimal detectable pressure (MDP) is 63.7 µPa/Hz1/2 @ 400 Hz. Due to its miniaturized structure and high sensitivity, the FOH may have an enormous potential in underwater target detection.